Current limiting fuse

ABSTRACT

A current limiting fuse element comprising a fuse link or portions of reduced cross section of such a relatively small area or size that they would normally be unable to support themselves alone. The fuse element is intimately bonded or attached to a relatively larger rigid dielectric mass by methods, such as printing, vacuum deposition or etching, so that the larger dielectric mass provides the structural support for the relatively thin current limiting regions or portions and also assists in removing heat from the fuse element during both normal operation and also during fusing of the fuse element. The fuse element and its dielectric support may be used independently or may be used as the fuse element in a cartridge-type fuse where the entire fuse element is disposed in a casing and embedded in a pulverulent, arc quenching material, such as sand, which may be provided to assist in arc extinction upon fusing of the fuse element and to absorb the heat during fusing. In addition, the relatively rigid, dielectric mass may be adapted to accommodate or receive a heat transferring means through which fluid may flow. Heat generated during normal operation of the fuse or during a fusion operation may be removed by the moving fluid.

[58] Field of Search ..29/623; 337/166, 185, 231

- United States. Patent [191 Blewitt [54 CURRENT LIMITING FUSE 75 items: Danna D'."waitresses; PaI

[73] Assignee: Westinghouse Electric Corporation, Pittsburgh, Pa.

22 Filed: Feb-16,1971

[21] Appl. No.: 115,197

5 [52] US. Cl. ..337/l66, 337/185, 337/231, 337/233, 337/295, 337/297 [51] Int. Cl. ..H0lll 85/02 HER PUBLICATIONS US. Dept. of Commerce, Miscellaneous Publication 192, lssuedNov. 22, 1948 ,pages 1 and 2.

[ Feb. 6, 1973 Primary Examiner-Bemard A. Gilheany Assistant E'xaminerF. E. Bell Attorney-A. T. Stratton and C. L. McHale [57] ABSTRACT A current limiting fuse element comprising a fuse link or portions of reduced cross section of such a relatively small area or size that they would normally be unable to support themselves alone. The fuse element is intimately bonded or attached to a relatively larger rigid dielectric mass by methods, such as printing, vacuum deposition or etching, so that the larger dielectric mass provides the structural support for the relatively thin current limiting regions or portions and also assists in removing heat from the fuse element during both normal operation and also during fusing of the fuse element. The fuse element and'its dielectric support may be used independently or may be used as the fuse element in a cartridge-type fuse where the entire fuse element is disposed in a casing and em- .bedded in a pulverulent, arc quenching material, such as sand, which may be provided to assist in arc extinction upon fusing of the fuse element and to absorb the heat during fusing. ln addition, the relatively rigid, dielectric mass may be adapted to accommodate or receive a'heat transferring means through which fluid may flow. Heat generated during normal operation of the fuse or during a fusion operation may be removed by the moving fluid.

2 Claims, 3 Drawing Figures I/IZO CURRENT LIMITING FUSE BACKGROUND OF THE INVENTION This invention relates to electric fuses and more specifically to current limiting fuses.

Current limiting fuses or fuse sections are often constructed by cutting or forming grooves, notches or other portions of reduced cross-section of various geometric shapes into fusible material which may initially be in the form of a ribbon or wire. The notches are arranged to create small cross-sectional regions in the fusible ribbon or wire. During normal operation, the fuse element or link conducts current through the small cross-sectional regions as the relatively larger adjacent regions or the regions where no grooves or notches have been cut serve to complete a continuous path for electrical current flow, and also to provide relatively larger surface areas for removal of heat which is generated or results in the smaller cross-sectional regions or portions. When overload current begins to flow in the fuse element, the notched sections or portions of relatively smaller cross-sectional areas melt to first form a plurality of relatively larger, spaced portions of electrical conducting material with a plurality of electrical arcs established between them. By carefully selecting the number of notched or smaller cross sections, a predetermined number of arcs may be established during the initial melting of the fuse element. Since each arc represents a regionof high resistance to current flow in the fuse element, the maximum current which flows is limited by providing a number of arcs.

Since the purpose of a current limiting'fuse is to prosectional area in the arc establishing regions must be relatively small. As a practical matter, even the strongest of metallic materials can only be reduced in crosssectional area to a certain lower limit or it will lose its structural strength in that it will be unable to support itself and may break during any handling or assembly. Correspondingly, the relatively soft materials, such as silver, used in making fuse elements cannot be notched or cut with too small a cross-sectional area or they will not support themselves at all. In addition, the added strain of supporting adjacent larger masses of fuse element material, such as those used for cooling or other purposes as was previously mentioned, also determines the size of the minimum cross-sectional area of the reduced cross-sectional regions. In the past as shown in U.S. Pat. No. 3,486,155, issued Dec. 23, 1969, fuse elements having a small cross-sectional area have been proposed which are supported by other means so that they will not disintegrate or break during normal operation or handling. In the latter patent, a thin ribbon of fusible material having a uniform thickness or width is deposited on or bonded to a plastic flexible material such as those sold under the trade-marks Mylar or Kapton." The Mylar plastic material and the fusible material are then wound around an associated core and inserted into a sand filled cylindrical tube to form a cartridge fuse. This construction is accomplished in a twostage process in which the fuse material is first bonded to the flexible tape-like material and then the tape-like flexible material is wrapped around or wound upon a ceramic base or core. In addition, the plastic strip in the latter patent is intended to be an insulating material so that the response of the fusible material is not affected by the presence of the strip material. It should also be noted that in the latter patent, that the flexible strip material may be subject to breaking, tearing, ripping or distortion.

It has also been proposed in U.S. Pat. No. 3,358,363 issued Dec. 19, 1967 to E. Jacks et al. to employ a method whereby fuses having very small cross-sectional notched areas could be made by a combination of a photographic and an etching process whereby an etchant-resistant layer coated with fusible material could be photographically treated in such a manner that when etching material was applied to the combination of fusible material and supporting layer, only that fusible material having the shape of the desired fuse element with a very small notch would remain after an etching process. In the last-mentioned patent, however, it was proposed that the supporting layer would then be dissolved away leaving a fuse element made only of fusible material.

It has also been proposed in U.S. Pat. No. 3,496,510, issued Feb. 17, 1970 to D. B. Hoover and in U.S. Pat. No. 3,465,275 issued Sept. 2, 1969 to W. Swain, to construct a fuse with an auxiliary brace or support of synthetic-resin-class-cloth laminate with the fuse material secured to the dielectric material by means of an eyelet or rivet. Fuse structures of the latter type do not provide the intimate bonding between the fuse element and the associated supporting material necessary to provide more efficient removal of heat between the parts or the possibility of creating an ultra thin fuse elementor section of precise geometry.

SUMMARY OF THE INVENTION In accordance with the invention, a fuse structure includes a relatively large dielectric or electrically insulating, rigid mass or a member which is formed from a suitable material and used as both a supporting means and as a heat removing means for an associated fuse element having a plurality of relatively smaller or reduced cross-sectional areas which melt and arc over in the presence of large values of overload current. The associated fuse element is secured to the relatively large dielectric mass or member by any one of a number of processes intimately bonding the fuseelement to the rigid dielectric member, such as vacuum deposition, printing or a combination photographicetching process with the fuse element sufficiently adhering to the dielectric mass to provide structural strength to the fuse element or link and to aid in the removal of heat from the fuse element.

It'is contemplated in one embodiment of the invention that the fuse element and its dielectric support will be assembled in a cartridge-type explosion proof fuse whereas in another embodiment of the invention, the combination of fuse element and dielectric support may be mounted in a fuse holder. Since the dielectric support as disclosed is relatively rigid compared with flexible plastic tape, the likelihood of the supporting material breaking, cracking, stretching or distorting under mechanical or thermal stress is greatly reduced, while the ability of the dielectric support to remove heat from the associated fuse element is increased. In addition, as with the cartridge fuse, a complete internal fuse section may be created in a simple one-step process whereby fuse material is bonded to the adjacent dielectric material and the combination is then placed internal to the cartridge fuse. This construction provides advantages over a fuse structure in which a fuse element is bonded to a flexible plastic strip and wrapped around a ceramic support or core. In another embodiment of the invention, the' relatively rigid dielectric support may be hollow or drilled in such a manner as to provide a passageway for cooling fluid to flow rapidly through it, thus increasing the heat removing capabilities of the entire fuse assembly such as would result when arcs are established during operation of a fuse structure.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention, reference may be had to the preferred embodiment exemplary of the invention shown in the accompanying drawings in which:

FIG. 1 is an orthogonal view of a fuse section embodying the invention.

FIG. 2 is an orthogonal view of a mounted fuse section similar to the one shown in FIG. 1 including a heat exchanging duct.

FIG. 3 is a view, in elevation, partially cut away, showing one embodiment of the invention as used in a cartridge or enclosed type fuse.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIG. 1 in particular, the fuse assembly shown therein comprises a uniquely shaped fuse element 14 intimately bonded or secured to a relatively rigid dielectric mass or support member 12. Dielectric member 12 may be formed from any electrically insulating material such as thermosetting resins of the phenolic, epoxy or polyester types with suitable fillers or ceramic type material such as alumina or beryllia provided it is relatively rigid and capable of supporting the associated fuse element or means 14 which is mounted thereon. Fuse element 14 may be formed from an electrically conducting fusible material, such as silver. It will be noted that fuse element 14 has a plurality of axially spaced, generally V- shaped notches, such as indicated at 18U and 18L, which may be oppositely disposed to form restricted portions such as indicated at 16A and 168, although any of the well-known restriction geometries or shapes as well as those not heretofore feasible may be used if desired. Restricted portions 16A and 16B are shaped to provide a plurality of portions having relatively smaller cross-sectional areas. During normal operating conditions, electrical current flows through the relatively larger sections 20 of the fuse element 14 as well as the notched or reduced sections 16A and 168. The area of notches 16A and 163 as shown in this example are designed to conduct a predetermined amount of high density electrical current without deteriorating. During normal operation, heat is generated in the vicinity of notches 16A and 168 due to the flow of currents I19 and I21 for example which may be components of a total current and are only represented as separate components to indicate the pinching effect on current in i the vicinity of each of the notches 16A and 168. The

heat generated in the reduced portions 16A and 163 must be dissipated, and the relatively larger fuse sections 20 act as heat sinks with the heat flowing or travelling outwardly from each of the notches 16A and 16B toward the adjacent sections such as 20A and 20B whereupon the heat is transferred both to the dielectric support 12 and to the ambient atmosphere. It is important to note that the relative size of the larger sections 20 and the effect of the dielectric support 12 which is bonded to fuse element 14 in an intimate manner, cooperate to efficiently dissipate the heat being generated in notches 16A and 168 which might otherwise cause fuse element 14 to melt and thus interrupt an associated electrical circuit even under normal operating current conditions.

Some of the suitable methods for intimately bonding fusible element 14 to dielectric support 12 are vacuum deposition of the fusible material, printing of the fusible material, pressing the fuse material 14 into the body of the dielectric mass or support 12 or a photographic masking and etching process such as that employed in preparing printed circuit boards. In the regions 16A and 168, it can be seen that in order to interrupt overload currents, the cross-sectional areas of the fusible material in the regions of notches or thin sections 16A and 168 must also be very small. However, there is a practical lower limit below which the cross-sectional area of fusible material may not be reduced and still support itself. This would occur if fuse element 14 were not supported by dielectric member 12 and if notches 18U and 18L were made very large so that bridging sections or narrow cross-sectional conducting sections 16A and 168 were extremely small so as to be able to interrupt very low overload currents. In such a situation, fuse element 14 would probably crack, break, bend, twist or shatter in the regions of notches 16A and 16B so that each relatively larger conducting section 20 would be spaced or separated from the adjacent conducting sections 20 even though no overload current had flowed to cause such a break. This of course would create an initially inoperative fuse and as can be seen the large weight and mass of sections 20 when compared with the small mass in regions 16A and 163 would tend to mechanically load or stress the thin bridging areas 16A and 16B and cause them to break or crack particularly during handling or assembly. However, it is important to note that it is necessary to keep sections 20 reasonably large to assist in conducting heat away from the reduced sections 16A and 16B. Consequently, in order to support a fuse element 14 from the structural standpoint, a rigid supporting or strengthening member formed from dielectric material as indicated at 12 is provided to insure that notches 16A and 168 can rely for structural strength on the adjacent intimately bonded or attached to dielectric member 12.

It is to be understood that the dielectric member 12 may be a rigid, relatively thin dielectric board or a much larger dielectric member with relative dimensions which differ from those indicated by the shape of the dielectric member 12 shown in FIG. 1.

Referring now to FIG. 2, another embodiment of the invention is illustrated in a fuse assembly including a heat conducting duct or pipe 32 having a right end 36 and a left end 38 which is disposed generally parallel to the fuse element 140 and'to fit into opening or hole 30 i of dielectric mass or support 120 so that a cooling fluid such as indicated at 34 may flow through pipe 32 and 1 further aid in removal of heat generated in fuse element 140 during either normal operation or during an overcurrent condition when the fusible material in regions {160A and 1608 heats up more rapidly than the fusible material in the adjacent sections 200. Asis known, the

initial melting of the material in the vicinity of the regions 160A and 160B create a series of arc voltages which in total act as a high resistance impedance to the flow of electrical current and thus limit the electrical current through fuse assembly 100 to a value less than" that to which the current might increase "due to the available current in the circuit to be protected. Fuse assembly 100 may be mounted in a supporting structure 39 having an electrically insulating base 40 withtwo electrically conducting fuse holders or clips 42L and 42R disposed thereon. Current may flow in a circuit 1 which extends from wire or lead 48R, fuse holder 42R to fuse element 140, at region 46 through fuse element 140 to region 44 and then through fuse holder 42L to lead or wire 48L. 1

Referring now to FIG. 3 another embodiment of the invention is illustrated in which a fuse assembly is mounted or disposed within a cartridge fuse 60. Fuse assembly 10 is secured at one end 68 to a generally cylindrical conducting means 66 and at the other end to a similar conducting means (not shown). Conducting means66 forms a closure means at one end of a cylinder 62 which is made of electrically insulating material. Cylinder or casing 62 is closed at the other end by the previously mentioned similar means. Fuse caps or terminals 64A and 64B are mounted at the insulating cylinder 62 to form contacts, adapted to fit an associated fuse holder. Consequently, an electrical circuit through which current may flow, extends from end cap 64A, through conducting section or member 66 into fuse element 14, through the other conducting member (not shown) and to the other end cap 648. In order to aid in arc extinction, a finely divided, pulverulent. or granular material 70, such as silica or quartz sand, may be disposed to substantially fill the space between the fuse assembly 10 and the cylinder. The material 70 acts as an absorber of thermal energy of arc currents when fuse element 14 is caused to melt or rupture due to overload current and it is also the absorber of heat generated during both steady state or normal operation and during an interrupting operation.

reduced portion formed by the removal of a generally circular piece of fusible material from the central por tion of the fuse element and a fuse portion formed by the removal of rectangular portions of fusible material to create rectangular notches, for example. It is also to be understood that although the fuse assembly of the invention is illustrated in two embodiments, one of which is a cartridge fuse embodiment, other embodiments of fuse sections are possible. It should be noted that heat removing section 32 of the fuse assembly 100 shown in FlG. 2, need not be of the particular form and shape indicated therein. It should also be understood that a plurality of fuse sections 20 may be connected in parallel circuit configuration.

The apparatus embodying the teachings of this invention has several advantages, for example a current limiting fuse with a plurality of current limiting sections may be used to limit or to protect against predetermined values of overload current, in applications where anunsupported fuse element made of the same fusible material would not be feasible because it would lack structuralintegrity. Also the adjacent dielectric material not only provides the structural strength required to It is to be understood that fuse element 14 may contain any number of reduced sections such as sections l6A'and 168, as determined by the voltage to be interrupted as well as other electrical characteristics. It is also to be understood that fuse element l4.with regions 16A and 1681 need not be formed only with generally rather than from disposed along the edges as illustrated i in FIG. 1. It is also to be understood that a fuse element may be provided with one fuse portion including smaller cross-sectional areas formedby two notches such as shown in FIG. 1, and another fuse element support the associated fuse element but aids in removing heat during both normal operation and during an interrupting or fusing operation from the fuse element. Also, the relatively rigid, adjacent dielectric member may be drilled or otherwise formed to carry a cooling or heat removing fluid away from the fuse element to further enhance the heat removing characteristics of the fuse assembly. Another advantage is that the dielectric member 12 is structurally strong and requires no further support or reinforcement in at least certain applications. Intimate deposition or bonding of fuse elements provide a means whereby a fuse element with a very small cross-sectional area or portion may conduct relatively high current density during normal or other than overload conditions without melting the adjacent sections of the fuse element and may more easily remove heat.

I claim as my invention:

1. An electric fuse structure comprising a substantially rigid dielectric heat conducting supporting member formed primarily from a thermosetting resinous material, a ribbon of fusible material having at least one notch to form a relatively smaller cross-sectional region intimately bonded along substantially its entire length to said dielectric member, said ribbon of fusible material being incapable of structurally supporting itself independently of the support of said dielectric member, said dielectric member including a separate integrally disposed tubular heat removing duct extending therethrough adapted toaccommodate the flow of heat absorbing fluid therein, a heat path existing from said fusible ribbon through said dielectric member into said tubular duct to the moving heat absorbing fluid.

2. An electric fuse member comprising a substantially rigid heat conducting dielectric supporting member formed primarily from a thermosetting resinousmaterial and including a generally tubular heat conducting duct therethrough adapted to carry a heat conducting fluid therethrough, a ribbon shaped fusible element intimately bonded along its entire length upon said supporting member by printing the fusible element thereon, said fusible element having at least one relatively smaller cross-sectional region which is incapable of structurally supporting itself independently of the support of said dielectric member, said intimate bond- 7 ing by printing facilitating the removal of heat generated in said fuse element during the operation of said fuse, said fusible element being generally parallel to said tubular duct. 

1. An electric fuse structure comprising a substantially rigid dielectric heat conducting supporting member formed primarily from a thermosetting resinous material, a ribbon of fusible material having at least one notch to form a relatively smaller cross-sectional region intimately bonded along substantially its entire length to said dielectric member, said ribbon of fusible material being incapable of structurally supporting itself independently of the support of said dielectric member, said dielectric member including a separate integrally disposed tubular heat removing duct extending therethrough adapted to accommodate the flow of heat absorbing fluid therein, a heat path existing from said fusible ribbon through said dielectric member into said tubular duct to the moving heat absorbing fluid.
 1. An electric fuse structure comprising a substantially rigid dielectric heat conducting supporting member formed primarily from a thermosetting resinous material, a ribbon of fusible material having at least one notch to form a relatively smaller cross-sectional region intimately bonded along substantially its entire length to said dielectric member, said ribbon of fusible material being incapable of structurally supporting itself independently of the support of said dielectric member, said dielectric member including a separate integrally disposed tubular heat removing duct extending therethrough adapted to accommodate the flow of heat absorbing fluid therein, a heat path existing from said fusible ribbon through said dielectric member into said tubular duct to the moving heat absorbing fluid. 